1. Field of the Invention
The present invention relates to a wireless power supply technique.
2. Description of the Related Art
In recent years, as a power supply method for supplying electric power to an electronic device, a wireless power supply method has been becoming popular. Such a wireless power supply method can be classified into two methods, i.e., the magnetic induction (MI) method and the magnetic resonance (MR) method. At present, as the MI method, (1) the “Qi” standard developed by the WPC (Wireless Power Consortium) and (2) the standard developed by the PMA (Power Matters Alliance) (which will be referred as the “PMA standard” hereafter) have become mainstream.
FIG. 1 is a diagram showing a configuration of a wireless power supply system 100R that conforms to the PMA standard. The wireless power supply system 100R includes a power transmitter (TX) apparatus 200R and a power receiver (RX) apparatus 300R. The power receiver apparatus 300R is mounted on an electronic device such as a cellular phone terminal, a smartphone, an audio player, a game machine, a tablet terminal, etc.
The power transmitter apparatus 200R includes a transmission coil (primary coil) 202, a driver 204, a controller 206, and a demodulator 208. The driver 204 includes an H-bridge circuit (full bridge circuit) or otherwise a half bridge circuit. The driver 204 applies a driving signal S1, and specifically, which is configured as a pulse signal, to the transmission coil 202. In this state, a driving current flows through the transmission coil 202. As a result, the transmission coil 202 generates an electric power signal S2 configured as an electromagnetic field signal. The controller 206 integrally controls the overall operation of the power transmitter apparatus 200R. Specifically, the controller 206 controls the switching frequency of the driver 204, or otherwise the duty ratio of the switching operation thereof, so as to change the transmission power.
The power receiver apparatus 300R includes a reception coil 302, a rectifier circuit 304, a smoothing capacitor 306, a modulator 308, a load 310, a controller 312, and a power supply circuit 314. The reception coil 302 receives the electric power signal S2 from the transmission coil 202. Furthermore, the reception coil 302 transmits a control signal S3 to the transmission coil 202. The rectifier circuit 304 and the smoothing capacitor 306 rectify and smooth a current IRX induced at the reception coil 302 according to the electric power signal S2, thereby converting the current IRX into a DC voltage VRECT.
The power supply circuit 314 charges an unshown secondary battery using the electric power supplied from the power transmitter apparatus 200, or otherwise steps up or steps down the DC voltage VRECT and supplies the DC voltage thus stepped up or down to the controller 312 or the load 310 configured as an external circuit.
In the PMA standard, a communication protocol is defined between the power transmitter apparatus 200R and the power receiver apparatus 300R. Such a communication protocol allows the power receiver apparatus 300R to transmit information to the power transmitter apparatus 200R in the form of the control signal S3. The control signal S3 is transmitted from the power reception coil 302 (secondary coil) to the transmission coil 202 in the form of an AM (Amplitude Modulation) signal using backscatter modulation.
The control signal S3 includes a power control signal (which will also be referred to as a “packet”) which controls an amount of electric power to be supplied to the power receiver apparatus 300R, and data which indicates the identifying information for the power receiver apparatus 300R. The demodulator 208 demodulates the control signal S3 included in the current or otherwise the voltage applied to the transmission coil 202. The controller 206 controls the driver 204 based on the power control signal included in the control signal S3 thus demodulated.
As a result of investigating the electric power control operation of the wireless power supply system 100R shown in FIG. 1, the present inventors have come to recognize the following problems.
In the PMA standard, the controller 312 of the power receiver apparatus 300R monitors the electric power supplied to the load 310, and generates, based on the monitoring result, a power control signal which indicates an amount of electric power to be supplied from the power transmitter apparatus 200. Specifically, a target value is set for the rectified voltage VRECT. Furthermore, an upper limit voltage VH and a lower limit voltage VL are set in the vicinity of the target voltage. The controller 312 generates a power control signal DPC such that the rectified voltage VRECT is positioned within a target voltage range REF (between VL and VH).
The PMA standard allows the power control signal DPC to switch between three states, i.e., (i) a state in which the transmission power is maintained (which will be referred to as the “first state ϕA”), (ii) a state in which the transmission power is increased (which will be referred to as the “second state ϕB”), and (iii) a state in which the transmission power is reduced (which will be referred to as the “second state ϕC”). The power transmitter apparatus 200R changes a transmission frequency fTX according to the power control signal DPC received from the power receiver apparatus 300, so as to control the electric power to be transmitted. Specifically, when the power control signal DPC is set to the first state ϕA, the transmission frequency fTX is maintained so as to maintain the transmission power. When the power control signal DPC is set to the second state ϕB, the transmission frequency fTX is changed by a predetermined width ΔfUP (e.g., by multiple steps) so as to increase the transmission power. Conversely, when the power control signal DPC is set to the third state ϕC, the transmission frequency fTX is changed by a predetermined width ΔfDN (e.g., by a single step), so as to reduce the transmission power.
FIGS. 2A and 2B are waveform diagrams each showing the electric power control operation of the power supply system 100R shown in FIG. 1. FIG. 2A shows the control operation in a non-oscillation state of the rectified voltage VRECT. At the time point t1, VRECT is lower than VL. Accordingly, in order to increase the electric power to be transmitted, the controller 312 switches the power control signal DPC to the second state ϕB. In response to this, the controller 206 of the power transmitter apparatus 200R changes the transmission frequency fTX by a predetermined width ΔfUP. As a result, the transmission power is increased, thereby increasing the rectified voltage VRECT.
When VRECT becomes higher than VH at the time point t2, in order to reduce the electric power to be transmitted, the controller 312 switches the power control signal DPC to the third state ϕC. In response to this, the controller 206 of the power transmitter apparatus 200R changes the transmission frequency fTX by a predetermined width ΔfDN. As a result, the transmission power is reduced, thereby reducing the rectified voltage VRECT. At the subsequent time point t3, the relation VRECT>VH remains. Accordingly, the power control signal DPC is maintained in the third state ϕC. In this state, the transmission power is further reduced, thereby further reducing the rectified voltage VRECT. By repeatedly performing such a control operation, such an arrangement is capable of stabilizing the rectified voltage VRECT in the target voltage range between VL and VH.
However, with the power supply system 100R shown in FIG. 1, it has been found that, in some cases, depending on the position relation between the transmission coil and the reception coil, the temperature, and the like, the rectified voltage VRECT cannot be stabilized within the target voltage range between VL and VH, i.e., falls into an oscillation state. Such an oscillation state occurs due to the fact that the relation between the variation ΔfUP (or ΔfDN) of the transmission frequency fTX and the variation in the rectified voltage VRECT is not constant, i.e., this relation changes according to the situation. Description will be made regarding the oscillation state with reference to FIG. 2B.
At the time point t1, VRECT is lower than VL. Accordingly, the power control signal DPC is switched to the second state ϕB. In response to this, the power transmitter apparatus 200R changes the transmission frequency fTX by ΔfUP so as to raise the electric power to be transmitted. This increases the rectified voltage VRECT by δVUP1.
When VRECT becomes higher than VH at the time point t2, in order to reduce the electric power to be transmitted, the controller 312 switches the power control signal DPC to the third state ϕC. In response to this, the power transmitter apparatus 200R changes the transmission frequency fTX by ΔfDN so as to reduce the electric power to be transmitted. This reduces the rectified voltage VRECT by δVDN2. At the time point t3, the relation VRECT>VH remains. Accordingly, the power control signal DPC is maintained in the third state ϕC. In this state, the transmission frequency fTX is further changed by ΔfDN, thereby further reducing the rectified voltage VRECT by δVDN3.
At the time point t4, the relation VRECT>VH remains. In this state, the power control signal DPC is maintained in the third state ϕC. Accordingly, the transmission frequency fTX is further changed by ΔfDN, thereby further reducing the rectified voltage VRECT by δVDN4. With (VH−VL) as ΔV, when δVDN4 is greater than ΔV, the rectified voltage VRECT cannot be stabilized within the target voltage range between VL and VH. Specifically, the rectified voltage VRECT becomes lower than the lower limit voltage VL. Such operations repeatedly occur, which leads to the rectified voltage VRECT falling into the oscillation state. Such oscillation is undesirable from the viewpoint of system stability. In addition, such oscillation leads to an increase in heat generation, resulting in a problem of degraded power transmission efficiency.
Such an oscillation problem is not restricted to the PMA standard. Also, such an oscillation problem can occur with other standards which will be developed in the future for providing the same electric power control operation.